1. Field of the Invention
The present invention relates to a process for covering silicas with wax and the use of such silicas.
2. Discussion of the Background
Matting agents for coatings based on wax-impregnated silicas (or silicic acids) are known and described in DE 10 06 100, DE 15 92 865 and EP 0 922 691, for example. Typically, a wax emulsion is converted with a silica suspension and, if required, a dispersing agent. The impregnated or covered silica thus obtained must then be dried at some expense and often still contains the dispersing agent. In addition, the binding of the wax to the silica is inadequate for many applications.
Another process for coating silica with wax is described in EP 0 442 325. Here, the silica is first impregnated with a polyol, which facilitates binding the hydrophobic wax to the hydrophilic silica. Coatings containing the impregnated silica according to EP 0 442 325 exhibit an undesirably high viscosity, however. Comparative examples in EP 0 442 325 indicate that silicas not coated with wax or coated with wax without the addition of polyol considerably reduce the viscosity of a coating. This is believed to be due to the excessively hydrophilic surface of the silica particles compared to the hydrophobic coatings. A polyol must be added in order to counteract this effect, which undesirably introduces an additional process step.
A thermal process for processing silica gels or silica sols is disclosed in DE 10 06 100. Here, silica hydrogels are first manufactured and dried, and activated in a further step. The dried gel is activated by heating it to about 468-538° C. and then converted with a molten, microcrystalline wax at 371° C., wherein 15-30% by weight of wax, relative to the hydrosol, is used. The very fine hydrosol particles exhibit a mesh-type gel structure with a very large water concentration and can only be covered satisfactorily at the above-described high temperatures or high wax concentrations. The high thermal load prohibits the use of many waxes, however (because of a temperature-induced change in the color of the wax); and rapid cooling of the resultant wax-covered silica gel to below the decomposition temperature of the wax must be ensured. Covering of silicas is not described in DE 10 06 100.
Silicas and silica gels exhibit different structures on account of the differing respective manufacturing processes. Silicas are manufactured by basic precipitation and exhibit an open particle structure. Water can be removed easily from these open-pored silica particles. In contrast to silicas, silica gels are obtained by acidic precipitation and have a rather closed structure from which water cannot be easily removed.
Accordingly, there remains a need for a process of covering silicas with wax, which allows for easy handling of the covered silicas and enables good sedimentation stability and dispersibility of the coated silicas, in coatings, for example, with minimal use of wax.